Aircraft typically include a flight control system for directional and attitude control of the aircraft in response to commands from the flight crew or an autopilot. A flight control system may include a plurality of movable flight control surfaces such as ailerons for roll control, elevators for pitch control, and a rudder for yaw control. The flight control surfaces may also include various devices for altering the lift and/or drag characteristics of the wings including leading edge devices (e.g., slats), trailing edge devices (e.g., flaps), and spoilers. Movement of the flight control surfaces is typically effected by actuators that may be mechanically coupled to the flight control surfaces. In many aircraft, actuators for flight control surfaces are hydraulically-driven by one or more centralized hydraulic systems which typically operate at a fixed operating pressure.
Hydraulic systems for aircraft are typically designed such that the flight control surfaces can be actuated in a manner allowing the aircraft to perform maneuvers covering all corners of the flight envelope, including outlier flight conditions. An outlier flight condition may require deflection of a flight control surface to a relatively high deflection angle during the failure of a hydraulic system powering one of the actuators. For example, an outlier flight condition may require deflection of the ailerons in a manner allowing the aircraft to achieve a relatively high bank angle within several seconds despite the failure of one of the redundant aileron actuators. Such a large deflection angle may require relatively high actuation forces in the actuators.
One solution for achieving high actuation forces for outlier conditions includes adding another actuator to the flight control surface. For example, in the above-noted example, instead of using two actuators on the aileron, a third actuator may be added such that a high actuation force may be generated by two of the actuators in the event that the third actuator is incapacitated due to a failure of one of the hydraulic systems. Unfortunately, the addition of a third actuator adds weight and complexity to the aircraft due to the need to route three hydraulic systems to the aileron. Another solution for achieving high actuation forces for outlier conditions includes using tandem actuators having multiple pistons, which unfortunately also adds weight and complexity to the flight control system. A further solution is to increase the working pressure of each of the hydraulic systems operating a flight control surface. Unfortunately, increasing the working pressure of a hydraulic system requires an increase in the size of all components of the hydraulic system which also adds weight to the aircraft.
As can be seen, there exists a need in the art for a system and method for increasing the actuation force capability of flight control surface actuators with a minimal increase in weight and complexity.